Dengue virus (DENV) infection has reached epidemic proportions in many countries in South America and Southeast Asia, with most experts advocating for a safe and efficacious vaccine to stop dissemination and reduce viral economic and disease burden. Advanced clinical trials of the Sanofi-Pasteur chimeric DENV- yellow fever virus (YFV) vaccine reported only partial protection and did not show efficacy against all DENV serotypes or in DENV-naive subjects. Most concerning, recent data has indicated that the Sanofi vaccine might enhance symptomatic disease in naive subjects, which has raised questions as to its eventual deployment. The lack of established immune correlates of vaccine-induced protection has hampered the development of a safe efficacious vaccine to DENV and to several other infectious diseases. Recent conceptual and technological advances in our understanding of effector mechanisms of cell and humoral immunity pave the way forward for implementing new assays that identify mechanisms and correlates of vaccine efficacy. A new live-attenuated DENV vaccine currently is being tested in Brazil, one of the countries where DENV has reached epidemic proportions. This TV003 vaccine formulation includes three attenuated strains corresponding to different DENV serotypes (DENV-1, DENV-3, and DENV-4) and a fourth chimeric strain (DENV-2/DENV-4), which contains the DENV-2 structural genes and the DENV-4 non-structural genes. One of the major advantages of the TV003 vaccine relative to the Sanofi DENV-YFV vaccine is that all of its components are from DENV strains. A Phase II trial of this vaccine has been completed and a Phase III trial has been initiated and will include over 17,000 subjects. Both phase II and III trials have been powered to include enough subjects to have training and test/validation cohorts. Peripheral blood mononuclear cells, serum and plasma have been collected on all subjects included in these two landmark studies. The major objective of our study will be to compare, prioritize and select the best assay that predicts the immunogenicity and the efficacy of this vaccine. We will test the hypothesis that several components (cellular, humoral and innate) are required to trigger the protection induced by this vaccine. The assays that will be tested here encompass a wide array of effector mechanisms of the cellular and humoral immune responses as well as novel genome-wide unbiased assessment of innate and adaptive immunity. A rigorous decision making tree will allow us in Aim 1 to identify and validate assays that predict a vaccine triggered broad immune response that targets all four DENV strains included in this vaccine. In Aim 2 we will identify the best in class assay(s) that predicts the efficacy of the vaccine in protecting recipients from infection. Efficacy will involve comparison of cases and controls if the vaccine's efficacy is below 80%; alternatively we will compare vaccinees and naturally infected placebos if the vaccine is highly efficacious (>85%). In Aim 3 our corporate partner Caprion, who has long standing experience in developing predictors of clinical outcome, will further optimize and scale the assay(s) identified in Aim 1 (immunogenicity) and in Aim 2 (efficacy) for robustness, accuracy, and throughput required for widespread clinical use in vaccine trials of DENV, and possibly other viruses.